1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to improvement of dimples of golf balls.
2. Description of the Related Art
Golf balls have numerous dimples on the surface thereof. The dimples disrupt the airflow around the golf ball during its flight to cause turbulent flow separation. By causing the turbulent flow separation, separating points of the air from the golf ball shift backwards leading to the reduction of drag. The turbulent flow separation prolongs the gap between the separating point on the upper side and the separating point on the lower side of the golf ball, which results from the back spin, thereby enhancing the lift force that acts upon the golf ball. Reduction in drag and elevation of lift force are referred to as “the dimple effect”. Excellent dimples disrupt the air flow more efficiently. Owing to the excellent dimples, great flight distance can be achieved.
In general, golf balls are formed using a mold having an upper mold half and a lower mold half. The upper mold half and the lower mold half each has a hemispherical cavity. Assuming that the upper mold half cavity is northern hemisphere of the globe and that the lower mold half cavity is southern hemisphere of the globe, an equatorial line will correspond to a parting line of the mold. Numerous protrusions are provided on the inner surface of the mold, and dimples are formed on the surface of the golf ball by means of the protrusions. The dimple has a shape reversed from the shape of the protrusion.
In molding, because the material (for example, synthetic resin) leaks outside from the parting line, a flash is generated along the equatorial line on the surface of the golf ball. This flash is ground and removed with a sand belt or the like. Because the dimples are recessed, removal of the flash generated inside the dimple is difficult. For ease in removal, no dimple is formed on the equatorial line. In other words, no protrusion is provided on the parting line of the mold. A great circle which does not intersect with the dimple (i.e., a great circle path) is formed on the equatorial line of the golf ball. When this great circle path agrees with a part where the greatest circumferential speed of the back spin is attained (hereinafter, referred to as “fastest part”), sufficient dimple effect cannot be achieved. Furthermore, the dimple effect achieved when the great circle path agrees with the fastest part is different from the dimple effect achieved when the great circle path does not agree with the fastest part. The difference between these dimple effects may deteriorate aerodynamic symmetry of the golf ball.
In light of the dimple effect, a mold having parting line with a concavo-convex shape was proposed. The golf ball obtained from this mold has no great circle path. This mold generates a flash having a concavo-convex shape. Grinding of the flash results in deformation of the dimple in the vicinity of the equatorial line. Thus deformed dimple cannot be responsible for the dimple effect enough. Also, this golf ball may not achieve sufficient dimple effect when the equatorial line agrees with the fastest part. This golf ball does not exhibit sufficient aerodynamic symmetry.
A regular polyhedron is often used for arranging the dimples. The regular polyhedron that is inscribed in the phantom spherical surface is envisioned, and edges of this regular polyhedron are projected on the phantom spherical surface by a ray of light emitted from the center of the sphere to the phantom spherical surface so as to form comparting lines. The comparting lines compart the phantom spherical surface, and the dimples are arranged. Examples of the regular polyhedron which may be used include regular hexahedron, regular octahedron, regular dodecahedron and regular icosahedron. In most common golf balls, a regular icosahedron has been used for arranging the dimples. The regular icosahedron results in formation of regular polygons in large numbers on the phantom spherical surface. The regular icosahedron achieves excellent uniformity.
WO99/11331 (JP No. 2001-514058) discloses a dimple pattern formed with a regular icosahedron. FIGS. 3 and 4 of this document illustrate a golf ball having 362 dimples. The surface of this golf ball can be comparted into twenty spherical regular triangles. Dimple patterns on all the spherical regular triangles are equivalent. This golf ball is formed using a mold the parting line of which has an concavo-convex shape. This golf ball does not have a great circle path.
The golf ball disclosed in the aforementioned document is not accompanied by any defect in aerodynamic symmetry resulting from the great circle path. However, difficulties may be involved in production of the mold in the case of this type of golf balls. Additionally, grinding of the flash may cause deformation of many dimples according to this golf ball. This golf ball does not solve the problem of defects in aerodynamic symmetry resulting from the grinding of the flash. There remains room for improvement of the aerodynamic symmetry of this golf ball. An object of the present invention is to provide a golf ball that is excellent in aerodynamic symmetry.